1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more specifically to a lithographic projection apparatus supporting assembly.
2. Background of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The projection system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extreme ultraviolet radiation (EUV), X-rays, electrons and ions. Also herein, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I-direction is denoted Ri. Furthermore, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface, rather than implying any particular orientation of the apparatus. Similarly, the term xe2x80x9chorizontalxe2x80x9d refers to a direction parallel to the substrate or mask surface, and thus normal to the xe2x80x9cverticalxe2x80x9d direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
EP-0,973,067-A discloses a supporting assembly for supporting a structure, such as the first or second object table or a reference, or metrology, frame, against gravity. The assembly comprises a piston associated with the supported structure and further comprises a cylindrical housing in which the piston is journalled. Gas bearings are provided in between the housing and the piston for providing a frictionless movement of the piston in its housing. The housing comprises a gas-filled pressure chamber and the gas in the chamber acts on the piston so as to counteract the weight of the supported structure. Gas from the pressure chamber is supplied to the gas bearing and may escape from the pressure chamber through a gap between the piston and its housing.
In the device described in EP-0,973,067-A it may be difficult to keep the pressure in the pressure chamber entirely constant. In practice, the pressure maintained in the pressure chamber will have a time varying component. This time varying, or dynamic component is due largely to the fact that gas is flowing from the pressure chamber to the various gas bearings. This flow induces pressure variations in the pressure chamber superimposed over the nominal static force which the pressure chamber must exert in order to counteract the forces of gravity due to the weight of the supported structure. The variations in chamber pressure due to the fact that gas must be supplied to the gas bearings lead to a dynamic pressure variation which can be seen as noise on the static force. This adversely affects the positioning accuracy of the positioning device.
According to one aspect of the present invention, there is provided a lithographic projection apparatus in which the supporting assembly is constructed and arranged such that substantially no gas flows through the pressure chamber when said moveable member is substantially stationary.
The apparatus of the present invention is constructed so that substantially no vibration forces are transmitted between the moveable member and its housing. This may be achieved by making them physically unconnected. However, in this case, there is necessarily a gap between the moveable member and its housing through which gas from the pressure chamber can escape. This is reduced in the present invention by ensuring that substantially no gas flows through the pressure chamber when the moveable member is stationary. This can be achieved in practice by providing a further gas supply to a further pressure chamber adjacent to the pressure chamber and at least partially surrounding the moveable member, in which the pressure is maintained so as to be substantially identical to that in the pressure chamber.
In one embodiment, the pressure chamber is supplied via a pneumatic resistor, which may include a small gap between the moveable member and a wall connecting the pressure chamber with the further pressure chamber. In this embodiment, the gas supply to the pressure chamber is provided from the further pressure chamber and passes through the pneumatic resistor gap.
Preferably, one or more gas bearings are located between the moveable member and its housing or between the moveable member and the supported structure. These gas bearings conveniently may be supplied via the further pressure chamber.
A gas cylinder as here referred to is sometimes also referred to as a (frictionless) pneumatic cylinder. By using the gas cylinder in the manner described above, the moveable member is supported by a constant pneumatic supporting force that is determined by gas pressure present in the pressure chamber. This gas pressure is not adversely influenced by flow induced pressure variations because the pressure chamber does not supply any flow to the gas bearings and escape of gas from the pressure chamber along the moveable member is prevented by the pressure of the further pressure chamber.
In one aspect of the invention, the gas cylinder, acting as a gravity compensator, functions by providing compressed gas (e.g. air or Nitrogen) which acts upon a cross-section of a moving piston with a fixed projected area in the vertical (or other intended) direction. This area can be provided by a single physical surface, but can also be distributed over a number of physical surfaces, or even be a differential area between two opposing surfaces. The counterbalance force provided by the pressure acting on this area should remain as near constant as possible, irrespective of horizontal, vertical, pitch, yaw or roll motion of the supported structure (e.g. mask or substrate holder), and its point of application should also remain static relative to the supportive part.
Also optionally in accordance with an aspect of the invention, the positioning means or device may be provided with, for example, three gas cylinders and three Z-Lorentz-force motors, each of the Z-Lorentz-force motors exerting, in operation, a substantially dynamic Lorentz force on the second part in the Z-direction, in parallel with the substantially static force provided by the gas cylinder. The three gas cylinders provide, in the Z-direction, a stable and statically determined support of the second part, e.g. against gravitational acceleration. By means of the three Z-Lorentz-force motors, the second part can be displaced in the Z-direction, and rotated about the first axis of rotation and the second axis of rotation. Since each of the gas cylinders can be incorporated as part of the Z-Lorentz-force motor, a practical and compact construction of the positioning device is obtained.
In an embodiment of a positioning device in accordance with the invention, a first part can be displaced relative to a base of the positioning device, at least in the X-direction, by means of a drive unit of the positioning device. In this embodiment, the first part can be displaced relative to the base of the positioning device over relatively large distances with relatively low accuracy by means of said drive unit, while a second part can be displaced with relatively high accuracy over relatively small distances relative to the first part by means of the system of Lorentz-force motors. As a result, the drive unit, which must have relatively large dimensions, may be of a relatively simple type with a relatively small positioning accuracy, while the dimensions of the relatively accurate Lorentz-force motors can be limited.
In a lithographic apparatus according to the invention, at least one of the object tables may be connected to a positioning device as described above, the substrate or mask holder being secured to the second part of the positioning device. The favourable properties of the positioning device in accordance with the invention manifest themselves in a particular way in the lithographic device in accordance with the invention in that transmission of mechanical vibrations from a supporting surface to the substrate or mask holder is precluded as much as possible. This has a favourable effect on the accuracy with which the substrate or mask holder can be positioned relative to the projection system, and on the accuracy with which the pattern or sub-pattern on the mask is imaged onto the substrate.
According to a second aspect of the present invention there is provided a supporting assembly including a moveable member associated with a supported structure, a housing in which said moveable member is journalled, said journalling being such that substantially no vibration forces are transmitted between said moveable member and said housing, a gas-filled pressure chamber, the gas in said pressure chamber acting on said moveable member so as to at least partially counteract the force due to the weight of the supported structure, and a gas supply for supplying gas to said pressure chamber, wherein, the supporting assembly is constructed and arranged such that substantially no gas flows through the pressure chamber when said moveable member is substantially stationary.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus including a radiation system for supplying a projection beam of radiation, a first object table provided with a mask holder for holding a mask, a second object table provided with substrate holder for holding a substrate, and a projection system for imaging irradiated portions of the mask on to target portions of the substrate, including, providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a mask that contains a pattern, projecting an image of at least part of the mask pattern onto a target portion of the layer of radiation-sensitive material, providing a moveable member associated with a supported structure, providing a housing in which said moveable member is journalled, said journalling being such that substantially no vibration forces are transmitted between said moveable member and said housing, providing a gas-filled pressure chamber, the gas in said pressure chamber acting on said moveable member so as to at least partially counteract the force due to the weight of the supported structure, supplying gas to the pressure chamber, and ensuring that substantially no gas flows through said pressure chamber when said moveable member is substantially stationary.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imagine step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), mutualization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices (dies) will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, when the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, waferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms maskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.